Implications of possible interpretations of 'greenhouse gas balance' in the Paris Agreement

Abstract

The main goal of the Paris Agreement as stated in Article 2 is 'holding the increase in the global average temperature to well below 2°C above pre-industrial levels and pursuing efforts to limit the temperature increase to 1.5°C'. Article 4 points to this long-term goal and the need to achieve 'balance between anthropogenic emissions by sources and removals by sinks of greenhouse gases'. This statement on 'greenhouse gas balance' is subject to interpretation, and clarifications are needed to make it operational for national and international climate policies. We study possible interpretations from a scientific perspective and analyse their climatic implications. We clarify how the implications for individual gases depend on the metrics used to relate them. We show that the way in which balance is interpreted, achieved and maintained influences temperature outcomes. Achieving and maintaining net-zero CO₂-equivalent emissions conventionally calculated using GWP₁₀₀ (100-year global warming potential) and including substantial positive contributions from short-lived climate-forcing agents such as methane would result in a sustained decline in global temperature. A modified approach to the use of GWP₁₀₀ (that equates constant emissions of short-lived climate forcers with zero sustained emission of CO₂) results in global temperatures remaining approximately constant once net-zero CO₂-equivalent emissions are achieved and maintained. Our paper provides policymakers with an overview of issues and choices that are important to determine which approach is most appropriate in the context of the Paris Agreement.This article is part of the theme issue 'The Paris Agreement: understanding the physical and social challenges for a warming world of 1.5°C above pre-industrial levels'.

Keywords: CO2 equivalence; Paris Agreement; emission metrics; greenhouse gas balance; net-zero emissions.

Figure 1.

Elements of Article 4 discussed in this analysis.

Figure 2.

(a) Range of permissible GHG emission scenarios for CO₂, CH₄, N₂O and F-gases for the 1.5°C and 2°C goals. (b) Example of aggregated emissions paths in terms of CO₂-equivalent emissions based on GWP₁₀₀ (SAR) indicating timing of net-zero CO₂-equivalent emissions. (c) Timing of net-zero CO₂-equivalent emissions as a function of metric type and time horizon.

Figure 3.

(a) Annual CO₂-fe emissions and (b) global mean temperatures for scenarios in the IPCC AR5 Scenario Database. CO₂-fe emissions have been calculated based on the total anthropogenic forcing in the Scenario Database. Temperatures are taken directly from the Database.

Figure 4.

Dependence of negative CO₂ emissions needed to balance residual emissions of CH₄ and N₂O (given as Mt CH₄ and Mt N₂O, respectively) on metric choice. For the left-hand panel, the bar height indicates the mean of the distribution of CH₄ and N₂O emissions in 2100, with the extent of the error bar marking the 5th–95th percentiles of the distribution. For the right-hand panel the bar heights indicate the mean net negative CO₂ emissions needed to balance the combinations of 2100 CH₄ (orange) and N₂O emissions (brown) present in the scenarios partitioned by metric (x-axis). The 5th–95th percentiles of the distribution of the total required net negative CO₂ emissions are marked by the vertical extent of the bars.

Figure 5.

(a) Global mean surface temperature response to maintained net-zero balance of CO₂-equivalent emissions from 2100 for various metrics. Thin lines indicate the range of response for the net-zero balances imposed using constant 2100 CH₄ and N₂O emissions from the complete set of 430–480 ppm scenarios, while thick lines indicate the response for the balance imposed from 2100 for the RCP2.6 scenario. (b) Calculations are done as perturbations above an RCP2.6 background of emissions with the perturbation implemented from 2100 and onwards. The spread in responses reflects the spread in non-CO₂ emissions due to scenario and model assumptions.

Figure 6.

Correlation between time of net-zero emissions and time of peak warming for various metric choices, including CO₂-fe. Grey dashed line represents the 1 : 1 line. Different colours represent the different metric choices, with open squares denoting the 430–480 ppm scenarios and filled symbols the 480–530 ppm scenarios.